ABSTRACT Ewing's sarcoma and related tumors are characterized by a specific t(11;22) chromosomal translocation fusing the amino-terminal portion of EWS with the DNA-binding domain of the ETS family transcription factor Fli1. The cellular origin for Ewing's sarcoma remains unknown at the present time, but there is evidence that the tumor may arise from either the mesenchymal or the neural lineage. It is not clear whether the EWS-Fli1 fusion protein alone is capable of cellular transformation and tumorigenesis. In addition, secondary genetic alterations such as INK4a deletion or p53 mutation have been frequently observed in Ewing's tumor specimen, raising the possibility that a second genetic hit may be critical to formation of Ewing's sarcoma. Since ubiquitous expression of EWS-Fli1 results in embryonic lethality, we have generated mouse lines harboring a conditional EWS-Fli1 allele that is normally silenced by a LoxP- flanked stop signal and can be activated in a cell type- or tissue-specific manner in the presence of Cre recombinase. In preliminary studies we have shown that EWS-Fli1 expression in primitive mesenchymal lineages causes developmental defects, especially in the skeletal system, but these mice are viable and grow into adulthood without any sign of tumor formation. The availability of these EWS-Fli1 transgenic mice and their inability to develop tumor from primitive mesenchymal tissues prompt us to test the central hypothesis that EWS-Fli1 is necessary but not sufficient for the formation of Ewing's sarcoma in vivo, and that the initiation and progression of Ewing's sarcoma requires an additional genetic alteration such as deletion of the INK4a tumor suppressor gene or mutation of the p53 tumor suppressor gene. To test this hypothesis, we will (I) generate mice in which expression of the EWS-Fli1 transgene is ubiquitous but is controlled temporally or in a tissue-specific manner, (II) examine the requirements for a secondary genetic alteration in the formation of Ewing's sarcoma in mice. The lack of a relevant animal model is clearly a major impediment to understanding the molecular pathogenesis of Ewing's sarcoma and has contributed to the difficulty in turning the discovery of the t(11,22) translocation into a clinically relevant, detailed understanding of the mechanism of transformation. Successful completion of our proposed work will help to reveal how initiation and progression of Ewing's sarcoma occurs in vivo in a defined genetic setting, thus provide a useful platform to test new treatment strategies for Ewing's sarcoma and related tumors.